Endometrial cancer (EC) is the fourth most frequent cancer in women, ranking first in incidence and second in mortality among female genital tract tumors. Although several genetic lesions associated with EEC have been identified, numerous grey areas still exist, especially regarding the mechanism of action of these mutations and their relative contribution to the progression from hyperplasia to metastatic cancer. A relationship between excess estrogen and type I EC (EEC) has been recognized for decades;however little is known about the molecular basis of this association. The broad, long- term objective of this project is to determine the role of the PI3K/PTEN/AKT pathway in endometrial hyperplastic and neoplastic transformation, and to elucidate the mechanisms relevant to these functions. As with most other neoplastic diseases, the elucidation of the pathogenetic basis of EC initiation and progression at the molecular level are greatly facilitated by laboratory models. Emerging reagents and strategies allow us to use the mouse in more and more sophisticated ways to define the molecular, cellular and physiological events that lead to cancer initiation and progression. We have developed a mouse strain that closely recapitulates several features of the progression from endometrial hyperplasia to neoplasia. We propose to utilize a combination of direct in vivo genetic approaches and ex vivo and in vitro experiments to carry out the following specific aims: Aim 1, To test the hypothesis that AKT activation in EEC causes a physiologically relevant alteration of ER1-mediated signaling. Aim 2, To identify and characterize, in mutant mice and cells, novel specific pathogenetic mechanisms activated by Pten loss during EEC initiation. The proper diagnosis and management of endometrial cancer are still major challenges in clinical oncology, as we still do not understand clearly the mechanisms that are altered during the progression from hyperplasia to cancer. This proposal is designed to dissect several specific biochemical and genetic pathways controlled by PTEN in its tumor suppressive function in the endometrial epithelium, and to define novel mechanisms involved in neoplastic transformation using engineered model systems that closely mimic the molecular events taking place in human lesions.